The present invention relates to pressing of materials to form pressed sheets of material, and more particularly to methods and systems for pressing lignocellulosic materials to produce composite wood products such as medium density fiberboard, oriented strand board and particle board.
There are many types of composite wood products formed from lignocellulosic material. Some examples include medium density fiberboard (xe2x80x9cMDFxe2x80x9d), oriented strand board (xe2x80x9cOSBxe2x80x9d), and particleboard. Others include laminated veneer lumber (xe2x80x9cLVLxe2x80x9d), laminated structural lumber (known in the art as xe2x80x9cGlulamxe2x80x9d) and plywood. These composite products are generally made by adhering together wood materials such as lumber, veneers, strands, flakes, particles, and fibers with adhesives. Thermosetting adhesives are commonly used, which include phenol-formaldehyde (PF) resins, phenol-resorcinol-formaldehyde (PRF) resins, resorcinol-formaldehyde (RF) resins, urea formaldehyde (UF) resins, melamine urea formaldehyde (MUF) resin and isocyanate (MDI) resin.
Such composite wood products are typically produced by adding the adhesive to the lignocellulosic material and forming a mat of the combination. The mat is heated to within the range of temperature within which the adhesive cures, and is pressed for a period of time, allowing the resin to cure completely, or nearly completely. This process is well known in the art as xe2x80x9chot pressingxe2x80x9d.
The resins within the mat need to reach a certain elevated temperature to cure properly, and it is necessary that the entire volume of the mat reach this certain temperature to allow all of the resin in the mat to cure. When heat is applied to the mat in a typical hot pressing operation, however, it is only applied to one or both face surfaces of the mat. In a typical system, the conveyor belt on which the mat rides is heated and supplies heat directly to the mat. The heat is expected to be conducted from the surface of the mat to the core of the mat. Since wood is not a good conductor of heat, however, this conduction of heat inwardly from the surface of the mat is an inefficient way of heating the core of the mat, and while the resin near the surface of the mat may reach its curing temperature relatively quickly, it takes much longer for the core of the mat to reach this temperature.
It is accordingly desirable to attempt to heat the core of the mat more quickly to save time and production cost in the manufacture of composite wood products. Attempts have been made in the past to accomplish this by adding more heat to pressing systems by way of steam injection, and also by way of subjecting the mat to microwave and radio frequency radiation. These methods are not desirable, however, because they require great amounts of energy, and, especially in the case of radiation, require significant protective mechanisms which are cost-prohibitive.
Accordingly, an improved method and system for heating the core of a mat of lignocellulosic material to allow shorter pressing times is still required.
In its most basic form, the present invention is a method for decreasing the time required to heat the core of a mat of lignocellulosic material to a temperature at which an adhesive resin in the mat can cure. The method includes the step of providing alternating or vibratory forces to a mat of material while it is being pressed.
In a preferred embodiment, the method comprises the steps of heating the surface of the mat from some initial temperature to a temperature within the temperature range at which the resin cures; for a period of time before the core of the mat reaches the temperature range at which the resin cures, subjecting the mat to a vibratory pressing force normal to the surface of the mat; and following the vibratory pressing force, subjecting the mat to a non-vibratory pressing force.
In a preferred embodiment, the step of subjecting the mat to a vibratory pressing force comprises alternatingly providing a first force to the mat and a second force to the mat, the second force being less than the first force. The frequency of this alternating force may be between 0.2 Hz and 2 Hz.
The invention also provides a system for accomplishing this method, the system having a press for accepting the mat; a temperature sensor for sensing the temperature of the core of the mat; a control system for accepting a first signal from the temperature sensor and in response to that signal causing the press to apply vibratory forces to the mat normal to the surface of the mat when the core of the mat is below a predetermined temperature. The control system is also programmed to cause the press to apply a non-vibratory force to the mat normal to the surface of the mat when the core of the mat is above a predetermined temperature, that temperature being within the range of temperature at which the resin in the mat is capable of curing.